The present invention relates to microparticles, preferably glass beads having an average diameter up to about 70 microns, which are coated to improve their dry flowability and to reduce their wettability within plastic compositions, which affects their dispersibility within plastic coatings, extrusions and molded products, and to methods for producing such coated particles and for producing plastic bodies containing such coated particles.
1. Field of the Invention
The present invention is concerned with improving the dry flow properties of microparticles such as glass beads, plastic beads, glass flakes, mica and similar pigment and color-enhancing materials. Particles of such materials normally have a surface affinity and attraction for each other, particularly in the presence of moisture, so that they have poor flowability properties in bulk, such as from within a container. This results in the particles forming clusters, agglomerates or build-ups of a plurality of flakes, beads or other particles, which interfere with their handling properties, metering properties and the aesthetic nature of the particles for their desired properties, such as color uniformity, light reflection or refraction and similar properties, as well as avoidance of significant reduction in impact strength caused by the addition of agglomerates to plastics. The present invention is further concerned with improving the dispersing, orienting or migration properties of the aforementioned microparticles within liquid plastic compositions, such as solvent coating compositions and molten extrusion and molding compositions, whereby the microparticles are not merely wetted and drawn by gravity down into the depth of the coating, extrusion or molded product.
2. State of the Art
It is known to add thickeners such as asbestine pigment to resinous paint solutions containing microbeads to prevent or retard settling during the spray-application of reflective highway paints, and to coat the beads with thin organophilic films to improve adhesion or affinity or wettability of the beads in the resinous binder material, and reference is made to U.S. Pat. No. 2,574,971.
It is also known to coat microbeads with adhesion promoters, including organosilanes such as 3-aminopropyltriethoxysilane or 3-methacryloxypropyltrimethoxysilane to insure that the microbeads are firmly secured to the substrate in a retroreflective screen printing ink having a resinous binder material. Reference is made to U.S. Pat. No. 5,650,213.
Also, U.S. Pat. No. 5,736,602 discloses the addition of colloidal suspending agents to curable thermosetting resinous coating compositions containing glass microspheres to retain the glass beads in suspension in the resin binder system.
However, the prior art does not disclose coating particles to render them repellent to each other, to avoid clustering and agglomeration and to improve flowability, and/or to make them repellent to resinous or plastic binder materials to improve their distribution and/or predetermine their location within resinous or plastic coatings, extrusions or molded products.
The present invention relates to the coating of microparticles such as glass beads, plastic beads, glass flakes, mica, pigments and similar color-enhancing particulate materials, particularly glass microspheres having average particles sizes up to about 75xcexc, preferably from about 1xcexc up to about 20xcexc, with materials which bond to the particle surfaces and impart free flowability to the particles in bulk and from their packages containing uniform dispersions of the particles in compositions, particularly compositions containing plastic or resinous binder materials for applying liquid coatings, or for extruding plastic rods, fibers or films, or for molding plastic bodies.
According to a preferred embodiment, the present invention relates to producing free-flowing self-repelling microparticles which can be easily compounded into conventional resinous or plastic compositions, without the need for thickening or viscosity-increasing additives, which particles are also repelled to predetermined and/or different degrees by compositions into which they are compounded, most particularly by the plastic or resinous binder materials thereof, to produce desired aesthetic or other results. One desired result can be to cause expensive microparticles to be concentrated adjacent the outer surface of the coating, extrusion or molded product to enable the use of smaller amounts of the microparticles, and/or to reduce surface imperfections such as flow lines during the extrusion or orienting of fibers or films.